Manufacturing B4C–(Al,Si) composite materials by metal alloy infiltration

The chemical interaction of porous B4C with molten Al during the infiltration step and the post-infiltrating heat treatment leads to the formation of a whole range of binary and ternary phases which affect directly the microstructure and the mechanical properties of the composite material. The formation of Al4C3 is undesirable because its presence causes a substantial decrease of the mechanical properties. In the present work, the effect of alloying aluminum with silicon on the formation of undesirable aluminum carbide in the infiltrated composites was investigated experimentally. The presence of Si improves the wetting and accelerates the penetration of the molten alloy into porous preforms. The addition of 40 wt.% Si to Al ensures that the final concentration of Si within the molten Al is higher than the threshold value (about 12 wt.%) that is necessary for preventing the formation of aluminum carbide in the B–Si–Al–C system. A significant increase of the hardness in the heat treated composites, infiltrated with Al–Si alloys, was observed and was attributed to the in situ formation of the very hard SiC phase and to the simultaneous formation of other ternary compounds.